The present invention relates to a pressure-heat drying method for controlling a drying process such that catalyst and the like may be uniformly deposited onto a workpiece made of porous material, such as ceramics, or metallic material, and also to an apparatus therefor.
When a workpiece to be dried, which has been coated with a solution containing catalytic substance dissolved in a solvent adhering thereto, is subjected to heat drying under atmospheric pressure, a temperature distribution within a vessel for accommodating the workpiece and applying the heat drying thereto may vary depending on locations therein and whereby the solvent adhering onto a surface of the workpiece does not evaporate uniformly, resulting in uneven deposition of the residual catalytic substance onto the surface of the workpiece having been subjected to the dry processing.
In order to cope with the above problem, there has been employed a conventional method in which a workpiece to be dried, which has been coated with a solution containing catalytic substance dissolved in solvent adhering thereto, is once subjected to an instantaneous freezing treatment to stop a movement of the catalytic substance (e.g., ions) and then the solvent is sublimated from the solution uniformly adhering to a surface of the workpiece in a solid state so as to dry the workpiece.
The above-mentioned freeze-drying method may be practiced by using, for example, a freeze-drying apparatus 50 shown in FIG. 8. To explain this method more specifically, a workpiece 60 with said solution adhering thereto is made frozen instantaneously by using liquid nitrogen or the like and brought into a vessel 51. Then, the vessel 51 is evacuated to vacuum (in the range of 10 Pa) by a vacuum pump 53 and the temperature in the vessel 51 is controlled so that it is not higher than a fusing point of the solvent (e.g., not greater than the fusing point of 0xc2x0 C. for a water used as the solvent) by a heater 52, so that the solvent adhering to the workpiece 60 to be dried may be sublimated.
However, said freeze-drying treatment is problematic in that the drying process could only take place moderately because of said treatment depending on the sublimation of the solvent adhering to the workpiece. This may lead to a problem whereby the completion of the drying process would take a considerably long period, a total of about one week. Besides, since the freeze-drying apparatus used therefor requires refrigeration facilities, vacuum evacuation facilities and so on in addition to the heating facilities, the entire unit of equipment must be large in scale and inevitably increase a cost thereof.
The present invention has been made in the light of the above-pointed problems, and an object thereof is to provide a pressure-heat drying method which allows a catalytic substance and so on to be uniformly deposited onto a workpiece to be dried in a dramatically short time period as compared to the prior art method and also a pressure-heat drying apparatus therefor.
The present invention provides a pressure-heat drying method for drying a workpiece to be dried in a pressure vessel, said workpiece coated with a solution containing a solute dissolved in a solvent adhering thereto, said method characterized in comprising: a pressure increase process for increasing a pressure in said pressure vessel to a set pressure higher than a saturated vapor pressure of said solvent at a set temperature; a temperature increase process for increasing a temperature in said pressure vessel to said set temperature under a condition where an evaporation of said solvent is suppressed with the aid of said set pressure which has been achieved through said pressure increase process; a pressure and temperature maintenance process for maintaining said set pressure and said set temperature in said pressure vessel at constant levels, respectively; and a pressure reduction and deposition process for reducing said set pressure to a level lower than the saturated vapor pressure at said set temperature and thereby stimulating a rapid evaporation of said solvent so as to allow said solute to be uniformly deposited onto said workpiece.
Further, an amount of evaporation of said solvent may be controlled by controlling a pressure reduction rate in said pressure reduction and deposition process.
The present invention further provides a pressure-heat drying method for drying a workpiece to be dried in a pressure vessel, said workpiece coated with a solution containing a solute dissolved in a solvent adhering thereto, said method characterized in comprising: a pressure increase process for increasing a pressure in said pressure vessel to a set pressure higher than a saturated vapor pressure of said solvent at a set temperature; a temperature increase, condensation and deposition process for increasing a temperature in said pressure vessel to said set temperature and thereby evaporating a certain amount of said solvent induced by a differential pressure between a saturated vapor pressure at a temperature of a cooling section in said pressure vessel and said set pressure in said pressure vessel to form a condensation thereof in said cooling section so as to allow said solute to be uniformly deposited onto said workpiece; a pressure and temperature maintenance process for maintaining said set pressure and said set temperature in said pressure vessel at constant levels, respectively; and a pressure reduction process for reducing said set pressure.
Further, an amount of said solvent to form the condensation in said cooling section may be controlled by controlling a temperature increase rate in said temperature increase, condensation and deposition process.
The present invention further provides a pressure-heat drying method for drying a workpiece to be dried in a pressure vessel, said workpiece coated with a solution containing a solute dissolved in a solvent adhering thereto, said method characterized in comprising: a pressure increase process for increasing a pressure in said pressure vessel to a set pressure lower than a saturated vapor pressure of said solvent at a set temperature; a temperature increase, evaporation and deposition process for increasing a temperature in said pressure vessel to said set temperature and thereby evaporating a certain amount of said solvent induced by a differential pressure between a saturated vapor pressure in said pressure vessel and said set pressure in said pressure vessel so as to allow said solute to be uniformly deposited onto said workpiece; a pressure and temperature maintenance process for maintaining said set pressure and said set temperature in said pressure vessel at constant levels, respectively; and a pressure reduction process for reducing said set pressure.
Further, an amount of evaporation of said solvent may be controlled by controlling a temperature increase rate in said temperature increase, evaporation and deposition process.
The present invention further provides a pressure-heat drying apparatus comprising: a pressure vessel for accommodating a workpiece to be dried, said workpiece coated with a solution containing a solute dissolved in a solvent adhering thereto; a pressure regulating means for increasing a pressure in said pressure vessel to a level equal to or higher than an atmospheric pressure by introducing an air or an inert gas into said pressure vessel; a temperature regulating means for increasing a temperature in said pressure vessel to a level equal to or higher than a room temperature; a pressure and temperature maintaining means for maintaining the pressure and the temperature in said pressure vessel constantly at a set pressure and a set temperature, respectively; and a pressure reducing means for reducing the pressure in said pressure vessel to the atmospheric pressure.
Alternatively, said pressure vessel may further comprise a cooling section and a temperature in the cooling section may be controlled to be equal or lower than said set temperature. Still alternatively, said pressure vessel may further comprise a circulation means for efficiently controlling the temperature in said pressure vessel. Yet still alternatively, said pressure vessel may accommodate a plurality of said workpieces.